PROPERTIES OF PORTLAND CEMENT. 401 



or no active unslaked lime when the test solution is applied. Hard- 

 burned, fresh cements often contain a high percentage of water which can 

 be expelled below 110 degrees. If such cements are stored in air-tight 

 receptacles it almost invariably happens that the percentage of this 

 constituent gradually decreases, but the decrease is extremely slow. We 

 can account for the slowness of this change by the fact that. the free lime 

 in the bard-burned cement is all sintered at a high temperature and 

 therefore it hydrates very slowly. One sample of such cement originally 

 contained 1.54 per cent of moisture and produced a fair amount of plume- 

 like crystals when the test solution was applied. The microscopic test, 

 from time to time, showed a gradual decrease in the quantity of plume- 

 like crystals and a corresponding increase in long, slender, radiating 

 needles. But only after nine months had the hydration of the sintered 

 lime become so complete that the microscope failed to show plumes. 

 The cement at that time contained 0.42 per cent of moisture. 



Clifford Richardson M suggests a different explanation for the changes noted 

 in cements which have been stored in air-tight receptacles. However, his theory 

 that the tension in solid solutions of calcium silicates and aluminates is released 

 by changes in temperature, etc., thus setting free some aluminate, is hardly 

 consistent with our knowledge of the behavior of solid solutions. It is unlikely 

 that a cement cooled from a white heat, aged, again heated during the grinding 

 process, and then slowly cooled in a pulverized state, would undergo any further 

 changes in its eutectie curve because of climatic changes in temperature alone. 



Table XII demonstrates that the amount of combined water which is 

 driven off during ignition remains fairly constant. All of the moisture 

 which combines directly with the lime is driven off at a low red heat. 

 The additional water of combination which shows a steady increase in 

 Table XII indicates that processes other than hydration of lime have 

 taken place, and that some hydraulic cementive action has set in. 



Although the duplicate samples checked very closely, Table XII shows 

 that brand D absorbed the least quantity of volatile .constituents. There- 

 fore, experiments were conducted to ascertain what effect the nature of 

 the cement would have upon" the rapidity and the amount of absorption. 



The rate of absorption decreases very rapidly as the reaction proceeds. 

 This is apparent from Table XII, but the cements there considered had 

 already been subjected to aeration. Table XIII records the figures ob- 

 tained by aerating a cement which previously had not been exposed to 

 the atmosphere. This material was ground from unaerated rotary 

 clinker, and before being exposed in the beakers its loss by ignition was 

 only 0.31 per cent. 



" Constitution of Portland Cement, read before the Ass. of Port. Cemt. Mfrs. 

 at Atlantic City, June, 1904. 



